Linking early determinants and cilia-driven leftward flow in left-right axis specification of Xenopus laevis: A theoretical approach

In vertebrates, laterality - the asymmetric placement of the viscera including organs of the gastrointestinal system, heart and lungs - is under the genetic control of a conserved signaling pathway in the left lateral plate mesoderm (LPM). A key feature of this pathway, shared by embryos of all non-avian vertebrate classes analyzed to date (e.g. fish, amphibia and mammals) is the formation of a transitory midline epithelial structure. Remarkably, the motility of cilia projecting from this epithelium produce a leftward-directed movement of extracellular liquid. This leftward flow precedes any sign of asymmetry in gene expression. Numerous analyses have shown that this leftward flow is not only necessary, but indeed sufficient to direct laterality. Interestingly, however, cilia-independent mechanisms acting much earlier in development in the frog Xenopus have been reported during the earliest cleavage stages, a period before any major zygotic gene transcription. The relationship between these two distinct mechanisms is not understood. In this review we present the conserved and critical steps of Xenopus LR axis formation. Next, we address the basic question of how an early asymmetric activity might contribute to, feed into, or regulate the conserved cilia-dependent pathway. Finally, we discuss the possibility that Spemann's organizer is itself polarized in the left-right dimension. In attempting to reconcile the sufficiency of the cilia-dependent pathway with potential earlier-acting asymmetries, we offer a general practical experimental checklist for the Xenopus community working on the process of left-right determination. This approach indicates areas where work still needs to be done to clarify the relationship between early determinants and cilia-driven leftward flow.